1. Field of the Invention
The present invention relates generally to hydrophones and, more particularly, to an enhanced sensitivity fiber optic interferometric hydrophone in which the path mismatch of the interferometer does not vary with changes in static pressure due to depth of operation.
2. Description of the Prior Art
Fiber optic interferometric hydrophones are well known in the art. They generally take one of two forms. In the planar hydrophone form, the hydrophones consist in part of fiber wound spirally into a disc form. In the linear hydrophone form of interest to this invention, the hydrophone consists in part of fiber wound around a cylindrical pressure-compliant sensor mandrel. Though many different materials may be used for the mandrel, the most sensitive designs use a hollow, sealed, air-filled mandrel. This fiber wrapped sensor mandrel is only part of the hydrophone. The complete hydrophone may take several forms.
One such form is the Michelson interferometer type hydrophone 20 shown in FIG. 1. The interrogating light signal is split by coupler 22 to two fiber optic branches. One branch is comprised of fiber wrapped sensor mandrel 24 previously described. Reflector 26 at the end of the signal fiber reflects light back though the sensor fiber to coupler 22. The other branch, or reference branch 28 is comprised of the same or nearly the same length of fiber wrapped in such a manner as to be insensitive to pressure. This branch is also terminated with a reflector 26 to reflect the light back to coupler 22. Sometimes the two wrappings are located concentric to each other, while in other designs they may merely be located near each other.
A second form is a Mach-Zehnder interferometer type hydrophone 30 shown in FIG. 2. This is similar to the Michelson type interferometer 20, except that the light is not reflected at the ends of the two branches, but recombined into a single path by a second coupler 32.
A third form is the pseudo-Fabry-Perot interferometer type hydrophone 40 shown in FIG. 3. Here, the sensor is comprised of only sensor mandrel 24 previously described, and two partial reflectors 42, one on either side of the mandrel. Partial reflectors 42 may be broadband partial mirrors or narrowband Bragg gratings. The second reference path is actually provided at a remote location in the transmitter/receiver equipment of the interferometer. There, a device known as a compensator splits pulsed light into two paths, delaying one set of pulses in relation to the other.
A primary disadvantage of previous types of hydrophones is that very expensive, low frequency noise lasers must be used to interrogate them. Less expensive lasers such as diode distributed feedback (DFB) lasers have higher levels of frequency noise, which limit the system noise performance. The system laser phase noise is proportional to the laser frequency noise and to the path mismatch length between the two branches of the interferometer. The Michelson type hydrophone generally requires a path mismatch of a few meters between the two paths of the sensor. The pseudo-Fabry-Perot type hydrophone can be precisely path matched to within a few millimeters. However, in all previous designs, static pressure changes occurring as the hydrophone changes water depth result in changes in the sensor path length and the mismatch. Such mismatches can be tens of centimeters, leading to unacceptable increases in laser phase noise.
A second disadvantage of previous designs of hydrophones is that they tend to be sensitive to a broad range of acoustic frequencies. In most applications, a restricted range of frequencies is of interest and the sensitivity to lower frequencies may cause problems, limiting performance in the band of interest.
The following U.S. patents describe various prior art systems that may be related to the above and/or other telemetry systems:
U.S. Pat. No. 4,525,818, issued Jun. 25, 1985, to Cielo et al, discloses an optical fiber hydrophone system in which a single optical fiber is used for all of the acoustical sensors in the system. A signal source and detector provides an optical signal in selected form, such as continuous or pulsed, and detects and extracts an identifiable output signal. Each sensor is in the form of a sensing portion of the single optical fiber. Each sensing portion includes two optical reflectors separated one from another by a predetermined length of said optical fiber. Variations in acoustical pressure incident on the sensing portion cause a change in the predetermined length. This causes reflected portions of the optical signal to interfere with one another. Such interference is detectable for extraction of the identifiable output signal. In one form, each sensing portion has two terminal branches of a mechanically deformable material, deformable in response to the fluctuations in acoustical pressure. Preferably, the optical fiber has two portions, a sensing portion thereof underwater and having a first optical cavity, and another portion thereof on board a vessel and having a second optical cavity, typically tunable with respect to the optical length thereof to maximize the interference in the detected optical signal.
U.S. Pat. No. 5,253,222, issued Oct. 12, 1993, to Danver et al, discloses an omnidirectional fiber optic hydrophone that includes a concentrically-arranged pair of ring-shaped mandrels mounted between planar upper and base members. Each of the rings is formed of inner and outer annular portions separated by an annular void. Optical fibers wound about the outer circumference of the outer annular portion of the outer ring and about the outer circumference of the inner annular portion of the inner ring communicate with a source of optical energy and with a photodetector to provide signals for measuring acoustic wave-induced deflections of the rings. A plurality of mandrels may be employed in a single hydrophone, which may be potted with elastomeric material or free flooded.
U.S. Pat. No. 5,317,544, issued May 31, 1994, to Maas et al, discloses a hydrophone that includes a plurality of hydrophone components separated by finite spacings and interconnected to provide a single output signal. Each hydrophone component is based upon a single-mandrel design in which a cylindrical body is apportioned into sensing and reference sections. The sensing sections comprise coaxial arrangements of pliant inner and outer cylinders separated by an annular airspace while the adjacent reference sections comprise solid-walled cylinders. Finite separation distances between the hydrophone components result in reduced flow noise occasioned by increased sensing area while detection sensitivity is maintained.
U.S. Pat. No. 5,394,377, issued Feb. 28, 1995, to vonBieren, discloses a hydrophone that is formed of first and second optical fibers coupled together to form a fiber optic interferometric sensor for sensing an acoustic signal. The optical fibers are wrapped around a pair of concentric, thin-walled hollow cylinders. The fiber wrapped around the inner cylinder is the reference leg of the interferometer and the fiber wrapped around the outer cylinder is the signal leg. The reference leg is exposed to the hydrostatic pressure but isolated from the acoustic signal. The sensing leg is exposed to both the hydrostatic pressure and the acoustic wave signal. The signal output from the interferometer is indicative of changes in the acoustic wave signal.
U.S. Pat. No. 5,504,720, issued Apr. 2, 1996, to Meyer et al, discloses a plurality of air-backed elongate mandrels that are arranged in an planar array such that their longitudinal axes are parallel. A length of a first optical fiber is wound around portions of each mandrel in a first group of the mandrels for exposure to the parameter. The first optical fiber is arranged such that exposing it to the parameter to be sensed causes the length of the first optical fiber to increase and decrease in direct proportion as the parameter increases and decreases. A length of the second optical fiber is wound around a second group of the mandrels for exposure to the parameter. The second optical fiber preferably is arranged such that exposing it to the parameter to be sensed causes the length of the second optical fiber to increase and decrease in inverse proportion as the parameter increases and decreases.
U.S. Pat. No. 5,668,779, issued Sep. 16, 1997, to Dandridge et al, discloses a hydrophone group for shallow towed applications in less than 50 feet of water. The hydrophone group has a series of hydrophones connected by relatively insensitive fiber optic interconnects. The individual hydrophones are sufficiently sensitive such that the interconnecting optical fiber does not introduce excessive noise. Each hydrophone is basically a sensing fiber wrapped around an air-backed mandrel. Each air-backed mandrel is formed of an extended solid frame substantially non-compliant along a longitudinal axis. The extended solid frame is provided with a channel around the periphery thereof. The channel extends substantially the entire length of the extended solid frame. A flexible outer covering surrounds the extended solid frame. The flexible outer covering is highly compliant in a radial direction extending from the longitudinal axis. The air-backed mandrel has a high frequency mechanical resonance. Each hydrophone is connected in a chain by a plurality of interconnects having a substantially lower sensitivity than each hydrophone.
U.S. Pat. No. 6,122,225, issued Sep. 19, 2000, to Cheng et al, discloses device for measuring pressure waves in a liquid medium that has a compensation chamber beneath the sensor. Cylindrical embodiments have an inner mandrel and an outer mandrel with a fiber sensor positioned between the two mandrels. A compensating chamber is defined between the inner mandrel and outer mandrel; however, the interior of the inner mandrel is filled with gas. No provision is made for equalizing the pressure of the inner mandrel with environmental pressure. It is presumed that at significant environmental pressures, the pressure inside the compensating chamber would act to compress the gas within the inner mandrel. The compressed gas in the inner mandrel would change the fiber sensor length and affect the path mismatch of the interferometer.
The above cited prior art does not disclose a hydrophone design which allows for path matching to be marinated over changes in static pressure and, additionally, provide enhanced sensitivity to allow high pressure usage. The solutions to the above described and/or related problems have been long sought without success. Consequently, those skilled in the art will appreciate the present invention that addresses the above and other problems.